Capacitors for use in electronic instruments such as cellular phone and personal computer are demanded to have a small size and a large capacitance. Among these capacitors, a tantalum capacitor is preferred because of its large capacitance for the size and good performance. In this tantalum capacitor, a sintered body of tantalum powder is generally used for the anode moiety. In order to increase the capacitance of the tantalum capacitor, it is necessary to increase the mass of the sintered body or to use a sintered body increased in the surface area by pulverizing the tantalum powder.
The method of increasing the mass of the sintered body necessarily involves enlargement of the capacitor shape and cannot satisfy the requirement for downsizing. In the method of pulverizing tantalum powder to increase the surface area, the pore size of the tantalum sintered body decreases and closed pores increase, as a result, impregnation of the cathode agent in the later step becomes difficult. As one of means for solving these problems, a capacitor using a sintered body of a material having a dielectric constant larger than that of tantalum is being studied. The material having a larger dielectric constant includes niobium and a niobium alloy.
With respect to the method for producing a niobium powder for capacitors, JP-A-55-157226 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses a method for producing a sintered element for capacitors by molding agglomerated powder into niobium powder having a particle size of 2.0 μm or less and sintering the niobium powder. However, details on the properties of the capacitor are not described in this patent publication.
U.S. Pat. No. 4,084,965 discloses a capacitor using a sintered body of niobium powder of 5.1 μm obtained by hydrogenating and grinding a niobium ingot. However, the capacitor disclosed is large in the leakage current (hereinafter sometimes simply referred to as “LC”) and the practicability thereof is low.
JP-A-10-242004 discloses a technique of partially nitriding niobium powder and thereby improving the LC value. However, when a high-capacitance capacitor is produced by using a niobium sintered body of niobium powder having a fine particle size, a capacitor having a peculiarly large LC value appears in some cases.